If hydrogen atoms are held close to one another in rigid or viscous systems, severe line broadening will result from their direct magnetic interactions (the spin-spin relaxation time, T 2, is short). The band separation and fine structure necessary to chemical structure determination cannot be observed under such conditions. These “direct dipole” interactions are eliminated when the hydrogen atoms are in constant rapid motion with respect to one another, leaving only the “indirect dipole” interactions which produce spin-spin multiplets. Consequently, the observation of the hydrogen resonance at high resolution requires that the sample molecule be in relatively free motion in all its segments. This usually means that the sample must be a gas or a liquid, although in a few cases useful spectra can be obtained from gels. The most convenient state is the liquid, so that both gases and solids are usually dissolved in suitable solvents for observation. Solids may also be melted.
KeywordsReceiver Coil Shift Reagent Exchangeable Hydrogen Sample Molecule Parent Band
Unable to display preview. Download preview PDF.